The present invention relates to sample collectors for sampling the particulate material in the atmosphere.
The determination of the degree of pollution in the atmosphere has received increasing attention in recent years. To make this determination, it is desirable to determine the number and kinds of particles in the atmosphere at various locations, and equipment has been provided for this purpose. Such equipment, known as particulate sample collectors, generally includes a filter, an apparatus for moving air through the filter, such apparatus normally including a duct in which the filter is placed, and a motor and blower, the motor actuating the blower to thereby cause the air to move through the duct. It is recognized that the particles which are collected by the filter tend to clog the filter, thereby gradually reducing the amount of air being drawn through the filter.
The particulate sample collectors are required to be placed in locations where they are subjected to various rigorous weather conditions. Being exposed to the atmosphere, they are subjected to rain storms, wind storms and temperatures from substantially below freezing to above 120.degree. F.
The problem of the gradual clogging of the filter has drawn the proposed solution of providing electronic circuitry to alter the flow rate through the filter as it becomes clogged; this presents the difficulty that the electronic circuitry is not reliable in all weather conditions, and is initially very expensive.
Particulate sample collectors heretofor known have provided a duct system in which the electric motor for driving the blower was placed. This electric motor was of the direct current type, having brushes, and it has been found that as the motor is operated, the brushes wear, emitting carbon into the atmosphere and some of this emitted carbon from the brushes of the DC motor had found to be recycled onto the filter, thus causing erroneous readings. For example, in a rural area, where 30 mg. has been collected on a filter, tests have indicated that as much as 28 mg. can be contamination from the brushes of the motor, and tests have indicated that whether the motor is up stream or down stream of the filter, the range of contamination from the motor has been from 2 to 28 mg., where the filter collected 30 mg. of such particles.
Yet another problem which has been found to have occurred in particulate sample collectors is that the rate of flow through the filter and duct has varied due to variations in the electric energy supplied to the electric motor, from conventional power lines. This has been a source of error in measuring the amount of air pollution or contamination.
Among the prior art known to applicant is the article "A New Flow Controller For High Volume Air Samplers" by Jerome L. Kurz and John. G. Olin of Sierra Instruments, Inc., Carmel Valley, Calif., presented at the 68th Annual Meeting of the Air Pollution Control Association, Boston, Mass., June 15-20, 1975; this paper describes a high volume sampler utilizing a hot wire filament with an electric feed back system to actuate a valve; this has such defects as noted above or being unreliable in some weather conditions and also being unduly expensive. In the article entitled "Sampling and Analysis of Atmospheric Sulfates and Related Species" by Robert K. Stevens and Thomas G. Dzubay, Atmospheric Environment, Volume 12, pg. 55, there is disclosed an apparatus for obtaining a constant flow rate in a sampler by utilizing a pressure regulator in the pump exhaust, the pressure regulator sensing the differential between atmospheric pressure and pressure in the exhaust line. The article "Constant Flow Regulators for the High-Volume Air Sampler" by Walter K. Harrison, Jr., John S. Nader and Frank S. Fugman, Journal of Industrial Hygiene, Vol. 50, page 574, discloses in FIG. 4 a constant flow regulator which is a variable restrictor placed in series in the flow line from the filter, a spring being utilized to counter-act pressure on a piston, to vary the outlet area as the flow rate would tend to decrease when the filter is clogged, and thereby maintain the flow rate constant.
Also of interest is "Aerosol Contamination from High-Volume Sampler Exhaust" by Ronald K. Patterson, APCA Journal, Vol. 30, No. 2, which illustrates a sampler which includes a motor within the duct system.
Baker et al U.S. Pat. No. 4,123,932 discloses an apparatus for monitoring a working area in which air is pumped through a so called "dosimeter"; as the filter becomes clogged, the flow rate is sensed by differential pressures across the discharge orifice from the pump, and a feed back loop, which includes electronic circuits, is used to control the speed of the motor which drives the pump. This apparatus, accordingly, requires a sophicated electronic circuit, which is expensive, and is the kind of apparatus which has not proven to be reliable in all weather conditions.
Jouault U.S. Pat. No. 3,438,179 provides a sampling apparatus including a blower for drawing air through a filter; down stream of the filter is a disc, to which oil is supplied. As the filter becomes clogged, less oil is fed to the disc, so that there is provided a constant flow rate over the entire sampling period. This apparatus requires a highly sophisticated arrangement, including a supply of oil and the partial clogging of a filter with the oil, and is therefore unsatisfactory for all air sampling uses.
Sehmel U.S. Pat. No. 4,159,635 discloses a filter in which a blower is down stream of the filter element. In this construction, a wind speed sensor controls an inlet opening to the filter and to the motor of the blower.
Boubel U.S. Pat. No. 3,841,145 discloses an apparatus for measuring particulate emissions in gas flows, and is used for sampling air over a very short period of time, such as one minute. This device is used, for example, for sampling gas in an exhaust stack and utilizes a manually operated exhaust valve.
Bosch U.S. Pat. No. 3,859,842 is another illustration of a gas sampler for an exhaust stack, and discloses the utilization of a diaphragm motor for controlling a butterfly valve in a by-pass conduit.